THE head of MI5 says Britain faces the worst terror threat in decades - with ISIS still a threat in Europe despite losing ground in the Middle East.

Britain's official terror threat level was raised to "critical" following the Parsons Green bomb attack in September but was later downgraded back to "severe". Here's the latest on the terror threat level and what the different classifications mean.

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Director General of MI5 Andrew Parker delivers a speech in central London, on the security threat facing Britain

What is the current UK terror threat level?

The current threat level for international terrorism in the UK is "severe", which means an attack is "highly likely".

There are five threat levels, decided by the Joint Terrorism Analysis Centre, designed to give a broad indication of the likelihood of an attack.

The UK's threat level was upgraded from "severe" to "critical" on May 23 following the Manchester Arena bombing the previous day.

Theresa May enacted the controversial measures of Operation Temperer amid the heightened alert, with troops deployed on the streets of Britain.

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Parker warned there are 3,000 people known to be involved in extremist activities

Police said a device - reported to be a Mother of Satan bomb - was placed inside a Lidl bag and left by the tube carriage door before exploding towards the rear of the train, filling the carriage with smoke.

Passengers were seen "running and screaming" from the tracks.

Many were trampled on as they tried to make their escape down from the elevated platform.

What happened in the London Bridge attack?

Seven people have died and at least 48 have been left wounded following the 10:08pm attack. Staff at a packed pub locked the doors as the gang tried to smash their way in. One man was stabbed five times.

Assistant Commissioner of Met Police Mark Rowley confirmed that all three of the depraved attackers were shot and killed within eight minutes of the first call to emergency services.